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Micromachines

Secure Transmission of Terahertz Signals with Multiple Eavesdroppers

Yuqian He, Lu Zhang, Shanyun Liu, Hongqi Zhang, Xianbin Yu

Published Aug 12, 2022• 11 citations
Publisher

Abstract

The terahertz (THz) band is expected to become a key technology to meet the ever-increasing traffic demand for future 6G wireless communications, and a lot of efforts have been paid to develop its capacity. However, few studies have been concerned with the transmission security of such ultra-high-speed THz wireless links. In this paper, we comprehensively investigate the physical layer security (PLS) of a THz communication system in the presence of multiple eavesdroppers and beam scattering. The method of moments (MoM) was adopted so that the eavesdroppers' channel influenced by the PEC can be characterized. To establish a secure link, the traditional beamforming and artificial noise (AN) beamforming were considered as transmission schemes for comparison. For both schemes, we analyzed their secrecy transmission probability (STP) and ergodic secrecy capacity (ESC) in non-colluding and colluding cases, respectively. Numerical results show that eavesdroppers can indeed degrade the secrecy performance by changing the size or the location of the PEC, while the AN beamforming technique can be an effective candidate to counterbalance this adverse effect.

DOI
10.3390/mi13081300

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The terahertz (THz) band is expected to become a key technology to meet the ever-increasing traffic demand for future 6G wireless communications, and a lot of efforts have been paid to develop its capacity. However, few studies have been concerned with the transmission security of such ultra-high-speed THz wireless links. In this paper, we comprehensively investigate the physical layer security (PLS) of a THz communication system in the presence of multiple eavesdroppers and beam scattering. The method of moments (MoM) was adopted so that the eavesdroppers' channel influenced by the PEC can be characterized. To establish a secure link, the traditional beamforming and artificial noise (AN) beamforming were considered as transmission schemes for comparison. For both schemes, we analyzed their secrecy transmission probability (STP) and ergodic secrecy capacity (ESC) in non-colluding and colluding cases, respectively. Numerical results show that eavesdroppers can indeed degrade the secrecy performance by changing the size or the location of the PEC, while the AN beamforming technique can be an effective candidate to counterbalance this adverse effect.
Terahertz (THz) band offers large accessible band-width and spatially confined beams for the realization of secure and high data rate communication. The paper aims to achieve security at THz band by considering a multiple-input-multiple-output (MIMO) wiretap channel. Source withMAM_{A}MA​antennas transmits using transmit antenna selection (TAS) technique and both main user and eavesdropper equipped with respectiveMMM_{M}MM​andMWM_{W}MW​antennas receive using maximal ratio combining (MRC) technique. In order to assess the secrecy performance, statistical characteristics are derived for the proposed system. Based on the derived statistical characteristics, the exact closed-form expressions for lower bounds for secrecy outage probability and probability of strictly positive secrecy capacity are derived. Results show that the improvement is oserved in the secrecy performance, which is primarily attributed to the utilization of transmit diversity techniques. It is demonstrated that non-linearity of the channel and multi-paths can also be exploited to enhance the physical layer security.
In this work, the secrecy performance of a multiple-input–multiple-output (MIMO) terahertz (THz) communication system is investigated. To enhance physical layer security (PLS), a novel transmission protocol is proposed, which dynamically tunes the carrier frequency to align with the centers of molecular absorption loss (MAL) lines. This frequency-selection strategy increases atmospheric attenuation, thereby limiting the signal’s effective range and making it significantly more difficult for a distant eavesdropper (Eve) to intercept the transmission. To assess the secrecy performance, the exact statistical characteristics of the signal-to-noise ratios (SNRs) at both the legitimate user and Eve are derived. Using these statistical characteristics, a unified analytical framework is developed, which yields closed-form expressions for key secrecy metrics, such as the average secrecy capacity (ASC), the lower bound on the secrecy outage probability (SOP), and the probability of strictly positive secrecy capacity (SPSC). Asymptotic analyses in the high-SNR regime further reveal the secrecy diversity order and secrecy coding gain, providing deeper insight into the system’s robustness. Numerical results show that the proposed MAL-aligned frequency tuning can improve ASC by up to 25% and achieves nearly a twofold reduction in insecure area compared to conventional fixed-frequency transmission. These gains are achieved while imposing only a marginal reduction in the legitimate user’s capacity, thereby maintaining an efficient balance between security and communication performance.
The future applications of terahertz (THz) wireless communication require investigations on link secrecy performance in all kinds of atmospheric conditions, including fog, snow, rain and atmospheric turbulence. Here, we present theoretical studies on physical layer security of point-to-point THz and infrared (IR) wireless links in atmospheric turbulence with a potential eavesdropper outside of the link path. Attenuations due to turbulence, gaseous absorption and beam divergence are included in the model to predict the propagation of both links. Secrecy capacity and outage probability of the THz links are calculated and compared with that of an IR (1550 nm) link. Dependences of link security on eavesdropper's position, atmospheric visibility, turbulence strength and intended data transmission rate are also presented and analyzed. We find that the THz link owns higher security at physical layer than the IR link.
Traditional cryptographic encryption techniques at higher layers require a certain form of information sharing between the transmitter and the legitimate user to achieve security. Besides, it also assumes that the eavesdropper has an insufficient computational capability to decrypt the ciphertext without the shared information. However, traditional cryptographic encryption techniques may be insufficient or even not suit- able in wireless communication systems. Physical layer security (PLS) can enhance the security of wireless communications by leveraging the physical nature of wireless transmission. Thus, in this thesis, we study the PLS performance in emerging wireless transmission systems. The thesis consists of two main parts. We first consider the PLS design and analysis for ground-based networks em- ploying random unitary beamforming (RUB) scheme at the transmitter. With RUB technique, the transmitter serves multiple users with pre-designed beamforming vectors, selected using limited channel state information (CSI). We study multiple-input single-output single-eavesdropper (MISOSE) transmission system, multi-user multiple-input multiple-output single-eavesdropper (MU-MIMOSE) transmission system, and massive multiple-input multiple-output multiple-eavesdropper (massive MI- MOME) transmission system. The closed-form expressions of ergodic secrecy rate and the secrecy outage probability (SOP) for these transmission scenarios are derived. Besides, the effect of artificial noise (AN) on secrecy performance of RUB-based transmission is also investigated. Numerical results are presented to illustrate the trade-off between performance and complexity of the resulting PLS design. We then investigate the PLS design and analysis for unmanned aerial vehicle (UAV)-based networks. We first study the secrecy performance of UAV-assisted relaying transmission systems in the presence of a single ground eavesdropper. We derive the closed-form expressions of ergodic secrecy rate and intercept probability. When multiple aerial and ground eavesdroppers are located in the UAV-assisted relaying transmission system, directional beamforming technique is applied to enhance the secrecy performance. Assuming the most general κ-μ shadowed fading channel, the SOP performance is obtained in the closed-form expression. Exploiting the derived expressions, we investigate the impact of different parameters on secrecy performance. Besides, we utilize a deep learning approach in UAV-based network analysis. Numerical results show that our proposed deep learning approach can predict secrecy performance with high accuracy and short running time.
Terahertz (THz) frequency band and free-space optical (FSO) communication can be proven potential solutions to the spectrum scarcity challenges in the emerging applications in the 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. It is assumed that both THz and FSO links encounter fading effects and pointing errors, where the THz link is modeled using α-μ distribution while Málaga ( M ) distribution is used to characterize the FSO link. Particularly, we conduct the physical layer security (PLS) analysis and derive the secure outage probability into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The effect of THz and FSO links parameters on the secrecy performance are then further investigated in detail. The results show that the security attacks on FSO links are more severe than those on THz links. Interestingly, the improved THz channel conditions adversely affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.
Terahertz (THz) frequency band and free-space optical (FSO) communication can be proven potential solutions to the spectrum scarcity challenges in the emerging applications in the 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. It is assumed that both THz and FSO links encounter fading effects and pointing errors, where the THz link is modeled using α-μ distribution while Málaga (M) distribution is used to characterize the FSO link. Particularly, we conduct the physical layer security (PLS) analysis and derive the secure outage probability into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The effect of THz and FSO links parameters on the secrecy performance are then further investigated in detail. The results show that the security attacks on FSO links are more severe than those on THz links. Interestingly, the improved THz channel conditions adversely affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.